Coaxial cables are a type of electrical cable used most oftentimes to carry high-frequency communication signals, e.g., signals that range from a fraction of a megahertz to tens of gigahertz in frequency. A typical coaxial cable includes a central conductor (or group of conductors), a dielectric insulator covering the central conductor, an inner cylindrical conducting shield or sheath (which is coaxial with the central conductor and which provides a signal reference or ground), and an outer insulating jacket. Ideally, the electromagnetic field carrying the signal exists only in the space between the central conductor and the inner shield, with the sheath reducing interference from external sources.
The dissipation factor of insulator material has a direct effect on the insertion loss results. In the case of coaxial cables, the lower the dissipation factor at frequencies greater than 1 GHz, the greater the performance levels. Dissipation Factor is expressed as the ratio of the resistive power loss to the capacitive power, and is equal to the tangent of the loss angle.
PTFE (polytetrafluoroethylene, e.g., DuPont Teflon®) is a synthetic fluoropolymer commonly used in the industry as the dielectric insulator in coaxial cables. PTFE insulators are implemented either in solid form or in expanded form, which is where air bubbles are incorporated into the PTFE material to lower its overall dielectric constant. PTFE has excellent electrical characteristics. However, as a thermoset material, PTFE cannot be melt processed, and is usually formed using a ram extrusion process. Here, a metering device is used to feed a measured amount of PTFE powder (paste) into a cylindrical extrusion pipe, where it is compressed by means of a hydraulic ram through an appropriately sized die onto a conductor.
The compressed PTFE powder/paste coated conductor is then transported through downstream ovens, where it is heated to dry off any extrusion aid and to sinter the PTFE insulation. This process can be effective for certain applications, but in the case of electrical cabling it is difficult to produce PTFE insulators with high dimensional tolerances, e.g., on a per-length basis, the thickness of the PTFE insulator may vary significantly. For high-frequency applications, such variances significantly negatively affect a cable's performance. Also, the PTFE ram extrusion process requires a large amount of machinery to carry out, and it is difficult to make lengthy continuous sections of electrical cable, since the sinter boundaries between rammed charges exhibit poor and/or variable electrical characteristics.
To overcome the aforementioned limitations of the prior art, it is a general object of the present invention to provide a coaxial cable having a central conductor group, an inner conductive sheath or shield coaxial with the central conductor group, and a high-purity FEP (fluorinated ethylene propylene) dielectric insulator disposed between the two. (Conductor “group” refers to one or more insulated or non-insulated conductors, including single and multiple solid conductors, stranded conductors, plated conductors, e.g., silver plated copper, and the like.) An outer insulator jacket and (optionally) an outer braided shield are disposed over the inner conductive shield. Although FEP is widely considered to be inferior to PTFE in the context of high-frequency coaxial and other electrical cables, the coaxial cable of the present invention utilizes an extruded, high-purity FEP material for the dielectric insulator. “High-purity” refers to FEP that is processed to have fewer impurities than a conventional FEP (and therefore a chemical structure that more closely approaches that of an ideal or theoretical FEP), and is defined as an FEP having a dissipation factor of 0.0005 or less at 2.45 GHz, as discussed in more detail below. Utilizing this type of insulator, the coaxial cable of the present invention is essentially equal to the electrical properties of a conventional coaxial cable having a PTFE dielectric insulator.